1. Field of the Invention
The present invention relates generally to a laser processing method of forming linear grooves by irradiating a thin-plate material such as e.g. a semiconductor wafer or the like with a laser beam. In particular, the invention relates to a laser processing method suitable for the case where grooves to be formed are intersected with each other.
2. Description of the Related Art
In the chip manufacturing step of a semiconductor device, the front surface of a generally disklike semiconductor wafer is sectioned into a large number of rectangular chip areas by predetermined dividing lines arranged in a lattice-like pattern. Electric circuits such as ICs, LSIs or the like are formed on the chip areas. Thereafter, the wafer is subjected to necessary processing such as rear surface grinding, etc., and then, the wafer is dividedly cut along the predetermined dividing lines, that is, is subjected to dicing, to provide the chip areas as semiconductor chips. The semiconductor chips thus obtained are packaged through resin encapsulation and widely used in various electric or electronic devices such as mobile phones, PC (personal computers) and the like.
Dicing means for dividing a wafer into individual semiconductor chips is heretofore generally blade dicing in which a disklike thin blade is rotated at high-speed and cut into the wafer. The blade dicing is said to have an advantage of providing a flat and sharp cutting surface. However, the predetermined dividing line between the chips needs to have a width greater than a size corresponding to the thickness (mainly about 10 to 30 μm) of the blade used. The necessity makes the cutting margin relatively large. Thus, the blade dicing is said to be disadvantageous in view of providing the number of chips per wafer as many as possible to improve productivity.
On the other hand, laser dicing has recently been adopted to subject a wafer to dicing by directing a laser beam capable of passing therethrough along a predetermined dividing line. Japanese Patent Laid-open No. Hei 10-305420 discloses a technology in which the wafer is irradiated from the front surface with a laser beam along a predetermined dividing line to form a groove and next cleaved along the groove thus formed, thereby providing chips. The formation of the groove by the removal of the front side of the wafer along the predetermined dividing line is due to the fact that the component of the wafer is heated by the laser beam irradiation to cause a thermal evaporation phenomenon called ablation. The laser dicing is said to be advantageous in view of productivity because a cutting margin is significantly smaller than that of the blade dicing.
In the dicing of a wafer, for example, respective grooves are usually formed in all predetermined dividing lines (first predetermined dividing lines) extending in one direction as described in the above-mentioned patent document. Next, respective second grooves are formed in second predetermined dividing lines perpendicular to the first predetermined dividing lines. Thus, the laser dicing will cause a state where a laser beam directed along the second predetermined dividing line is allowed to perpendicularly intersect the first groove formed in the first predetermined dividing line. In this state, a portion, of the diced chip, immediately before the laser beam directed along the second predetermined dividing line reaches the first groove is influenced by overheat comparatively different from those of other large portions in some cases.
As shown in FIG. 7, examples of such an influence by the overheat include deformation in which the width of a portion immediately before a second groove G2 reaches a first groove G1 is enlarged so that respective corners of chips 3 on both sides of the second groove G2 are each not be formed at a right angle. In FIG. 7, an arrow indicates the traveling direction of the laser beam directed to the second predetermined dividing line. In addition, the corner not formed at a right angle becomes fragile to lower bending strength, which may cause a crack or damage in some cases. Further, such a corner has an influence on the easiness of picking up each chip 3 after the completion of dicing.
The easiness of pickup is explained. When the wafer is diced, to handle a wafer hard to be transferred, the wafer is affixed to a dicing tape and a dicing frame affixed to the dicing tape is handled. After completion of the dicing, chips are peeled off from the dicing tape and picked up. In this case, if having higher peel property, the dicing tape is preferably peeled with ease. Because of this, the dicing tape uses a UV (ultraviolet) cure adhesive, and when the chip is picked up, the dicing tape is irradiated with ultraviolet to lower the adherence of the adhesive, thus making pickup easy.
However, during the dicing, the portion immediately before the laser beam directed along the second predetermined dividing line may be overheated as described above. In such a case, the ultraviolet cure adhesive of the portion becomes modified so that adherence is not lowered even by the irradiation of ultraviolet. If such a thing occurs, a portion of the adhesive being left with adherence even through the overheat adheres to the chip during the pickup, which will cause a failure hard to pick up the chip. This failure occurs even in the case where processing by laser beam irradiation is groove formation or where the wafer is full-cut, i.e., fully cut. Incidentally, the alteration and the lowering of the bending strength described above are likely to occur during the full-cut but less frequently occur during the groove formation.